Software driven predictive circuit modeling such as Simulation Program with Integrated Circuit Emphasis (SPICE) is known. SPICE software is typically used as an aid by circuit designers to simulate the operation of a given circuit. If the simulation indicates that the circuit exhibits suboptimal performance, then the step of circuit fabrication is not performed. By not building and testing a circuit on a circuit substrate, a circuit designer can save a substantial amount of time and money pursuing an improved design with a higher likelihood of desired performance. Predictive modeling permits an iterative circuit design process as the designer optimizes the performance of the circuit according to desired performance parameters. Multiple simulations using slight component value variations and/or designs can aid a circuit designer's understanding of how manufacturing tolerances of component values affect performance of the circuit. Predictive modeling can also be very powerful as an aid to simultaneous engineering where different parts of a system are designed and built concurrently. Predictive modeling can be used to increase the likelihood that the entire system will perform as intended. Concurrent engineering and manufacturing significantly reduces time to market and reduces the high costs of scrap, rework and redesign.
An accurate predictive model is of significantly greater value than one that is less accurate. The better the model can simulate the performance of the fabricated circuit, the higher the confidence level of decisions made based on the simulated performance. In prior art predictive models, the electrical performance of circuit interconnects including cables, wires, traces and stripline is largely ignored. Where logic switching time is long relative to the transmission time through the interconnect, the idealized simulation of instantaneous and complete transmission is an adequate assumption. As frequency increases, however, the instantaneous transmission assumption is not adequate. At higher frequencies, an actual transmission line exhibits not only propagation delay but also frequency dependent attenuation and dispersion largely due to skin effect.
There are known lossy transmission line model elements which are modeled as a series circuit of multiple lumped filter sections. One such lossy transmission line element is known as the U model and is part of HSPICE.RTM. software version H92 available from Meta-Software Inc. of Campbel, Calif. In the U-model, the lumped element values are calculated given certain geometric and structural parameters for known transmission structures. From the structural parameters, the U-model calculates values for a series combination circuit comprising a resistor and inductor in series with a capacitor in parallel to a reference point. The U-model also includes a conductance in parallel with the capacitor and may include two or more stages of the combination circuit. Typically, these models include a dc resistance and an elevated resistance to simulate attenuation at a higher frequency. In the case of digital signaling, however, there is a broadband of frequencies of interest that comprise an excitation signal having a given rise time. Accordingly, a single resistance parameter to model higher frequency attenuation results in an incomplete model. Disadvantageously, the results of this simplified lossy line may be inadequate in that it does not accurately simulate broadband transmission line performance. There is a need, therefore, for a frequency dependent lossy transmission line model element for use in circuit simulation systems that can more accurately predict transmission line performance over a broad range of frequencies improving digital signaling predictive performance. An improved circuit simulation system when used in conjunction with a circuit fabrication process can significantly reduce the time and cost of bringing an electronic product to market having desired operational performance.